1. Field of the Invention
The invention relates to the technical field of image sensing and, more particularly, to an exposure control system and method for an image sensor.
2. Description of Related Art
In using an image sensor, a shutter is typically arranged in front of the image sensor to control the exposure amount and time for the image sensor. Such a shutter can be a mechanical or electronic shutter. In order to meet with compact size requirement, light and low cost requirement for a product, the electronic shutter has become the preferred device equipped with the image sensor.
The electronic shutter uses an internal electronic signal of the image sensor to control the entire exposure timing and procedure. In a typical CMOS image sensor, the operations of reset, exposure and read of its internal devices are in a row-by-row manner. As shown in FIG. 1, a row will be reset before to expose the row of pixels. To have an identical exposure time for each row, consecutive rows are reset at a constant rate. Next, the electronic shutter at each row is opened/closed sequentially for exposure. Such a shutter is referred to as a rolling electronic shutter.
Accordingly, an illuminator requires being kept in an open state to provide the required light source for the exposure procedure of all rows. As a result of a long exposure time, the illuminator cannot use a high-power light source otherwise the illuminator would be burned. Namely, the illuminator consumes a smaller current, such as 150 mA, between T3 and T4. However, even though the illuminator consumes the smaller current, the exposure time is long so that the entire power consumption is relatively high. To reduce the power consumption, a brightness of the illuminator is reduced. However, such reduction may require a closer distance between the illuminator and an object surface. Thus, the irradiated angle cannot be increased effectively. In addition, the electronic shutter at each row of the CMOS image sensor needs to prolong its open time to obtain enough illumination. Unfortunately, such a way may easily result in an image blur effect because of the long exposure time. Conversely, if the exposure time is reduced, the CMOS image sensor requires an enlarged sensitive area in order to obtain enough light.
To overcome the problem, U.S. Pat. No. 2003/0007088 A1 discloses that a multi-flashlight is used in an exposure of the image sensor to thus obtain an identical averaged exposure amount for each row of pixels of the image sensor. As shown in FIG. 2, the first flashlight illuminates only on rows 0-11, not on rows 12-31. Also, the second flashlight influences only on rows 12-23, not on rows 0-11 and 24-31. In this case, each row receives only one flashlight so as to have identical exposure amount for each pixel from row 0 to row 31. However, when an object is moving between T1 and T2, its image may present a cracked effect.
Therefore, it is desirable to provide an improved system and method to mitigate and/or obviate the aforementioned problems.